Within the National Monitoring of Contaminants in Biota, Stockholm University has analysed chlorinated pesticides and PCBs (CLC), brominated compounds (BFR) and perfluoroalkyl substances (PFAS) since the start of the time period (the earliest samples from the end of 1960s). However, from 2021 and 2022 the analyses will instead be performed by the Swedish Food Agency (CLC, BFR) and Swedish University of Agricultural Sciences (PFAS). Therefore, there is a need for comparisons between the laboratories in order to investigate if conversion factors will be needed for any of the substances. Thus, the aim of this report was to compare concentrations of CLC, BFR and PFAS analysed at different laboratories.Samples from fish (marine and freshwater), blue mussels and guillemot eggs were analysed at the different laboratories. Thereafter concentrations were compared using t-tests and conversion factors were calculated.Results for CLCs and BFRs showed, with the exception of BDE-100 and BDE-154, that differences were small between Stockholm University and Swedish Food Agency.We therefore recommend that no conversions should be used for the CLCs or BFRs, but suggest a re-evaluation of BDE-100 and -154 at a later stage.For most PFASs too many data points were below LOQ for reliable conversion factors to be calculated. Differences were seen between Stockholm University and Swedish University of Agricultural Sciences for l-PFOS, b-PFOS, and PFOS (sum of l-PFOS and b-PFOS) when considering all samples. However, the samples were run in two batches at Stockholm University (separated time-wise, but also from two different environments – freshwater and marine). When splitting the data into these two batches, only one showed a significant statistical difference between the labs. Due to the few number of samples, the possible batch effect and the relative small difference between labs (l-PFOS: 23% for all samples combined), it is not recommended to apply conversion factors.In this report, we conclude that no conversion factors will be used to adjust the data from the national monitoring programmes for contaminants in marine and freshwater biota.